The present invention relates to a method for tuning a summing network of a base station, which summing network consists of connectors, conductors and a filtering means which include input connectors for receiving signals supplied by radio transmitters of the base station, and output connectors for feeding the filtered signals further to an antenna means. The invention further relates to a bandpass filter comprising an input connector, an output connector and a resonating means.
The invention particularly relates to a summing network for combiner filters in a base station of a cellular mobile communication network. A combiner filter is a narrow-band filter which resonates exactly on the carrier frequency of a transmitter coupled to it. The signals from the outputs of the combiners are summed by the summing network and fed further to the base station antenna. The summing network usually consists of a coaxial cable leading to the base station antenna, to which coaxial cable the combiner filters are usually coupled by T-branches. In order that as much as possible of the transmitting power of the transmitters can be forwarded to the antenna, the summing network should be tuned with regard to frequency channels used by the transmitters of the base station. Thus, the optimal electric length of the summing network is dependent on the wavelength of the carrier wave of the signal to be transmitted. Strictly speaking, a summing network is thereby tuned on one frequency only, but the mismatch does not at first increase very fast when the frequency changes away from the optimum. Thus, base stations of cellular communication systems can usually use the summing network on a frequency band whose width is approximately 1-2% of the center frequency of the frequency band used by the base station. This sets very high requirements for the mechanical length of the summing network and its cabling, because the transmission lines must be of precisely the correct length in order for the summing network to be optimized on the correct frequency. In addition, the usable frequency band of a summing network is too narrow for the frequency channels of the base station transmitters to be changed very much without having to deal with the tuning of the summing network. As especially such combiner filters that are automatically tuned (by remote control) have become more common, need has arisen for simple and fast change in the tuning of the summing network. The prior art solution, according to which it was necessary for an engineer to visit the base station site and to replace the summing network cabling with a new cabling measured for the new frequency band, is understandably too expensive and time consuming a procedure.